Lili 
gradient may be increasing below. 
For many purvoses one can neglect the details of the equilib- 
rium very near the sea surface, which are affected by the appreciable 
depth of nenetration of solar radiation, so it is only necessary to 
consider the net effect on the water density of the transfer of heat 
and water across the surface. In the absence of frictional mixing 
and of horizontal flow, if this net effect is such as to decrease 
density, the stability is increased and, if heat is added also, the 
temperature gradient becomes more negative. However, if this net 
effect is such as to increase density, the nearly isothermal layer 
increases in depth. On the other hand, when the surface layer is 
frictionally stirred with sufficient activity, the water and heat 
exchange does not alter its nearly neutral equilibrium and its 
nearly isothermal character. 
The effects of solar heating, nocturnal radiation, conduction 
to or from the atmosphere, and evanoration will be treated indi- 
vidually below. The presentation is aimed to make vossible a quan- 
titative evaluation of each from the common meteorological observa- 
tions obtained on shipboard,especially tovard obtaining the net ef- 
fect of all four. Precipitation is omitted from individual atten- 
tion because quantitative observations of it are not commonly avail- 
able and usually it is sufficient to consider only its cualitative 
effect. 
Vapor pressure 
The vapor pressure in the air nlays an important part both in 
radiation processes and in evaporation. 
By way of introduction it may be recallcd that, if a pan of 
